Presently, there is much activity in the development of spread spectrum communications systems in both the commercial and military arenas. A spread spectrum communication system is a system in which a plurality of communication units (radios) communicate with each other over a wide band of frequencies within a single communications channel. As a result, no one frequency is dedicated to any one communication network. This frees-up air-space so that a greater number of systems can use the limited number of available frequencies for communication over the air. Consequently, spread spectrum systems provide a more economical solution for over-the-air multiple access communications.
One technique for implementing spread spectrum communications is frequency hopping. In a frequency hopping system, the carrier frequency shifts from frequency to frequency in a predetermined pseudo-random pattern throughout the spectrum of the communication channel at a predetermined time based on the network clock. The network clock is established when the local clocks of all the units communicating on the channel are substantially synchronized to the same time. Without this local clock synchronization or network time, the units communicating on the channel will not hop to next frequency at the same time, and thus will lose communication with each other.
Essentially, frequency hopping acts as a time-frequency coding technique that provides a high degree of protection from frequency jamming as well as protection from eavesdropping on the communications channel. As a result, its success is directly dependant on the accuracy of the network timing. Since the network time is based on the local clocks of all the units communicating on the channel, it is not dependant or linked to the actual time of day.
One method of establishing and maintaining an accurate network time between the units communicating on the channel is disclosed in Pat. No. 5,121,408, entitled "Synchronization For Entry To A Network In A Frequency Hopping Communication System," issued Jun. 9, 1992, to Cai et al, and incorporated herein by reference. Cai et al discloses a synchronization arrangement in which each unit on the network is initially synchronized to the same time or initial network time. As communications take place, each unit continually tracks its local clock deviation from that of the network time, making any corrections necessary to maintain synchronicity between its local clock and the network clock. This synchronization process, which is inherent to all present day FH communications systems, is crucial to maintaining communications with the network.
Some frequency hopping systems initially set the network time to the actual time of day by preloading each unit intending to communicate on the channel with that time. The units on such a system usually have a built-in network time correction or update procedure that keeps its local clock in sync with the local clock of all the other units on the network. The network time update or correction to each local clock is usually done at the beginning of every transmission.
Basically, when a unit starts to transmit, it sends its local clock time to all the other receiving units. The receiving units, in turn, compare this time to their own local clock time. If there is a difference between the two times, the receiving unit calculates a tracking adjustment time by which it changes its local clock time to be as close to the transmitted time as possible. Thus, it can be seen that the tracking adjustments are only as accurate as the tracking adjustment time calculated by the receiver. Moreover, since the transmitting unit dictates the update time, it also dictates network time during its transmission. As a result, many clock or network time corrections may occur during extended communications involving many transmissions, and thus result in a network time drift away from the real time of day.
This network time drift is not detrimental to those units that maintain constant communications with the network because they will always hop to the correct frequency at the same time as all the other network units that have similarly drifted with the network clock. The network time drift, however, will adversely affect those units that lose communication with the network as well as those units wishing to establish communication with the network after the network has drifted some critical amount of time. This critical time is a function of the accuracy of the network time correction function or minimum adjustment time of each unit, and essentially indicates how far a local clock can drift from the network clock before resynchronization becomes impossible. Thus, each communications system has a different critical time outside which a remote unit can not drift if it wants to maintain communications with the network.
This network time drift phenomenon has been observed in the United States Army's Single Channel Ground and Airborne Radio System (SINCGARS). In SINCGARS, it has been observed that the network clock loses time relative to real time at a rate whose probability is proportional to the rate of transmissions between the units on the communications channel. As a result, after extensive transmissions between units which are synchronized with the network clock, it is difficult for a late coming subscriber or a subscriber who loses communications with the SINCGARS network to join the network and synchronize its local clock with the drifting network clock. The network operator has to use additional late network entry procedures, and thus is delayed in joining or rejoining the network.
Moreover, a problem arises when the transmitting unit begins its transmission just before a new frequency hop. When this happens there is a large probability that all the receiving units will decode the network clock information, sent by that transmitting unit, during the following network time slot. As a result, all the receiving units will think they are in the wrong time slot and thus adjust their local clocks backward to the preceding time slot. Thus, the entire network time or network clock will drift back one unit of time or one frequency hop.
It can thus be easily seen that over extended communications the network time will be pulled backwards by these local time adjustments before each transmission. This adjustment time is called the tracking adjustment time. Consequently, the critical parameter for each unit is its minimum tracking adjustment time. This is the minimum amount of time a network unit can adjust its local clock to be in line with the network time as dictated by the current transmitting unit. As a result, the minimum tracking adjustment time dictates the network clocks accuracy and drift amount during extended communications having many transmissions.
To reiterate, if this minimal tracking adjustment time is large enough to change the network clock to the preceding time slot or preceding frequency hop, then the network will essentially drift backwards with respect to real time. As described above, this will hinder those units that lose communications with the network and those units that later wish to join the network from joining or rejoining the network.
As demonstrated in the SINCGARS system, the most common method of reducing the probability of network time drift, due to these minimal adjustments, is to maintain a "master" unit on the network to independently keep the network clock. The master unit periodically transmits the network time so that each unit can maintain synchronization between its local clock and that of all the other units on the network without having to resynchronize during each transmission.
Using a master unit, however, presents many problems in the field. For one, if anything happens to the master unit such that it can not maintain communications with the other network units, the network clock will drift as described above. Secondly, the constant transmission of network time information from the master unit will reduce the efficiency or throughput of the communications channel. Lastly, not all spread spectrum communication systems that utilize the frequency hopping technique have the option of using a master unit for network timekeeping.